1. Field of the Invention
The present invention relates to an optical measuring apparatus which enables a coordinate positioning machine (such as a machine tool) to determine the position of an object relative to a reference point. It may, for example, be employed on a machine tool for toolsetting operations.
2. Description of Related Art
A known-tool setting device for use on a machine tool includes a light source which generates a fine beam of light which is incident upon a detector. During a toolsetting operation, the machine is operated to move the tool in a direction transverse to the direction of propagation of the light beam until a part of the tool interrupts passage of the light beam. Detection of this interruption is used to produce a trigger signal in the detecting unit, which is used by the machine to establish the relative position of its moving parts in order to determine dimensions of the tool. Such devices are known, for example, from German Patent Nos. DE 42 385 04 and DE 42 448 69, French Patent No. 2,343,555, European Patent No. 98,930 and U.S. Pat. No. 4,518,257. The devices may be used additionally for measuring the length or diameter of a tool to monitor tool breakage or wear.
The devices disclosed in the above-mentioned patent specifications use a narrow light beam into or through which the tool is passed. The detection units detect when the tool breaks into the beam from the resulting drop in the intensity of the light falling on them. The trigger signal may be produced as a result of a predetermined drop in the intensity of light falling on the detector as the tool enters the beam.
A problem which arises with such optical measuring apparatus is that coolant used on the machine can drip through the beam, or be thrown off the rotating tool into the beam, during the measuring operation and give rise to false trigger signals.
One method of overcoming this problem which is currently used, is to program the software in the machine controller to perform several measurements until a pre-selected number of measurements falling within a given tolerance have been obtained. The position of the tool is then assumed to be the average of these measurements. This method can give rise to an unacceptable increase in the measurement cycle time if a significant number of repeat measurements have to be taken.